An inverter is relatively common in a modernized power system. A direct current port of the inverter is connected to an external direct current power supply, and an alternating current port of the inverter is connected to an alternating current power grid. A main function of the inverter is to convert direct current electric energy to feed alternating current electric energy that has a constant voltage and a constant frequency and feed the alternating current electric energy to a power grid.
In an operating process, an operating status of the inverter is affected by an actual condition of the power grid. When the inverter is connected to the power grid using a long-distance electricity transmission line, equivalent impedance of the power grid cannot be ignored. The equivalent impedance of the power grid affects voltage stability of an alternating current port of the inverter, and further affects the operating status of the inverter. When the equivalent impedance of the power grid seriously affects the inverter, the inverter cannot operate stably. A typical unstable phenomenon is voltage or current oscillation, and a more serious phenomenon includes shutdown of an inverter or a power supply system caused by triggering protection.
Therefore, before the inverter enables a power transfer mode, impact of the power grid on the inverter needs to be analyzed. The power transfer mode refers to that the inverter converts direct current electric energy to alternating current electric energy that has a constant voltage and a constant frequency and feeds the alternating current electric energy to the power grid. There is a common test method that can be used to test the impact of the power grid on the inverter. Before the inverter enters the power transfer mode, a measurement apparatus injects a disturbance current into a measured system, and calculates a ratio of a voltage to a current of a port of a measured module to obtain a needed equivalent impedance matrix Z of the power grid. After obtaining data of the equivalent impedance matrix Z of the power grid, and of an equivalent admittance matrix Y of an inverter port, a product of Z and Y may be calculated. According to a stability criterion of generalized Nyquist, when and only when a quantity of times that an eigenvalue trace of the product of Z and Y bypasses a point (−1, 0) counterclockwise in a complex plane equals a quantity of right half plane poles of the product of Z and Y, the power grid has relatively little impact on the inverter, and the inverter can operate stably.
However, in an actual measurement process, a power level of the measurement apparatus needs to match a power level of the measured module. Therefore, hardware costs of the entire system are increased. Common measurement apparatuses include a switched capacitor resistor network, an inverter, a synchronous motor, a linear power amplifier circuit, a network analyzer, and the like. When a connection point between the inverter and the power grid is relatively remote, using the measurement apparatus may significantly increase transportation and maintenance costs. When the connection point between the inverter and the power grid is varied, a position of the measurement apparatus also needs to be changed accordingly. In this case, extra transportation costs are generated. In addition, each of transportation, installation, and operation of the measurement apparatus consumes a specific time, thereby affecting operation efficiency of the entire power grid.
It may be learned from the foregoing that, before an inverter performs power transfer, analyzing impact of a power grid on the inverter is quite necessary. However, currently, there is no exact method for determining impact of the power grid on inverter stability when measurement costs are reduced.